1. Field of the Invention
Embodiments of the invention described herein pertain to the field of electric submersible pump assemblies. More particularly, but not by way of limitation, one or more embodiments of the invention enable an electric submersible pump assembly bearing.
2. Description of the Related Art
Fluid, such as gas, oil or water, is often located in underground formations. In such situations, the fluid must be pumped to the surface so that it can be collected, separated, refined, distributed and/or sold. Centrifugal pumps are typically used in electric submersible pump (ESP) applications for lifting well fluid to the surface. Centrifugal pumps impart energy to a fluid by accelerating the fluid through a rotating impeller paired with a stationary diffuser. A rotating shaft runs through the central hub of the impeller and diffuser. A motor upstream of the pump turns the shaft, and the impeller is keyed to the shaft, causing the impeller to rotate with the shaft.
Each rotating impeller and stationary diffuser pair is called a “stage”. The impeller's rotation confers angular momentum to the fluid passing through the pump. The angular momentum converts kinetic energy into pressure, thereby raising the pressure on the fluid and lifting it to the surface. Multiple stages of impeller and diffuser pairs may be used to further increase the pressure lift. The stages are stacked in series around the pump's shaft, with each successive impeller sitting on a diffuser of the previous stage.
A conventional ESP assembly includes, from upstream to downstream, a motor, seal section, intake section, and multi-stage centrifugal pump. Production tubing carries the pumped fluid from the centrifugal pump to the well's surface. The assembly components each have a shaft running longitudinally through their centers that are connected and rotated by the motor. In gassy wells, a gas separator or charge pump may also be included in the assembly. For example, a gas separator may act as the intake of the assembly. In such instances, the gas separator compresses the gaseous fluid and then attempts to separate any unsaturated gas before the fluid passes into the centrifugal pump. Gas separators sometimes include impeller and diffuser stages to increase the pressure of the fluid during compression and separation of gases. Similarly, charge pumps are also sometimes used in tandem with a primary centrifugal pump in gassy wells, and may also employ stages.
During operation, whether in a pump, charge pump or gas separator, pump assembly stages are subject to axial forces in the upward and downward directions, conventionally referred to as “thrust.” Downward force or “downthrust” is a result of a portion of the impeller discharge pressure acting on the top of the impeller. Upward force or “upthrust” is a result of a portion of the impeller discharge pressure acting against the bottom of the impeller. A second upward force is the force produced by the momentum of the fluid making its turn in the impeller passageway. Pump assembly stages are also subject to radial forces that can cause the shaft to become misaligned.
To carry the thrust of the pump, thrust bearings are sometimes employed in pump stages. The thrust bearings include a bushing that is pressed into the wall of the diffuser. A sleeve is keyed to the shaft inward of the stationary bushing. A thin layer of fluid forms in between the sleeve and bushing of the bearing set to provide fluid film lubrication and carry the downthrust loads. The sleeve may further act as a radial support bearing.
Stationary bushings are traditionally disk-like in shape or shaped like a hollowed cylinder. A conventional stationary bushing is illustrated in FIG. 1. The conventional bushing must remain stationary as the sleeve rotates, and as such, these bushings must be securely pressed into the inner wall of the diffuser exit. To make room for the bushing, the diffuser core is bored to remove material from the wall of the diffuser exit, and in doing so, the area of the diffuser exit normal to absolute flow velocity is reduced. Reducing the area of the diffuser exit reduces the performance of the stage by diminishing the ability of the stage to increase the pressure of the fluid passing through the pump assembly.
Typically, the reduced performance caused by insertion of the bushing is combatted by increasing the number of pump stages, which adds cost to the pump assembly. In addition, the bushing is conventionally made of a hard material like tungsten carbide, which is expensive and so the bushing itself adds to the overall cost of the pump assembly.
As is apparent from the above, current ESP bearings suffer from many shortcomings. Therefore, there is a need for an improved electric submersible pump assembly bearing.